Tuft cells.

Tuft cells, also known as brush cells, are widespread in the hollow organs of the digestive tract including the duct system of the salivary gland and in the respiratory tract, from simple vertebrates to humans. The shape of tuft cells varies from pear-shaped, to barrel-shaped and goblet-shaped, apparently depending on the plane of section. The most characteristic morphological features of tuft cells are their long and blunt microvilli, which have prominent rootlets, and a well developed tubulovesicular system in the supranuclear cytoplasm. Both the microvilli and tubulovesicular system can be labeled with lectin and periodic acid-thiocarbohydrazide-silver proteinate-physical development (PA-TCH-SP-PD), suggesting a relationship between them. Many spheres observed among the microvilli seem to originate from the head of a polyp-like structure protruding into the vesicles, suggesting some type of apocrine secretion. During mammalian development, tuft cells increase around the time of weaning as neonates gradually become accustomed to solid food. Tuft cells in the rat gallbladder and stomach possess intermediate filaments, that is, neurofilaments and cytokeratin-18 filaments. Despite numerous morphological studies, the functions of tuft cells are still obscure. The discovery of the presence of alpha-gustducin has provided a clue to the long-sought function of tuft cells, which appear to possess the cellular and molecular basis for chemoreception. The present review discusses the three currently proposed functions of tuft cells--secretory, absorptive and receptive--on the basis of morphological, histochemical and cytochemical evidence.